1. Field of the Invention
The present invention relates to a membrane electrode assembly including a first electrode, and a second electrode, and a solid polymer electrolyte membrane interposed between the first electrode and the second electrode. Further, the present invention relates to a fuel cell including the membrane electrode assembly.
2. Description of the Related Art
For example, a solid polymer fuel cell employs a membrane electrode assembly (MEA) which includes an anode and a cathode), and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane (proton exchange membrane). Each of the anode and the cathode is made of electrode catalyst and porous carbon. The membrane electrode assembly and separators (bipolar plates) sandwiching the membrane electrode assembly make up a unit of a fuel cell for generating electricity. A predetermined number of the fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hydrogen-containing gas) is supplied to the anode as a reactant gas. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen or air (oxygen-containing gas) is supplied to the cathode as a reactant gas. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
In the fuel cell, a fuel gas flow field (reactant gas flow field) is formed in a surface of the separator facing the anode, and an oxygen-containing gas flow field (reactant gas flow field) is formed in a surface of the separator facing the cathode. Further, a coolant flow field is formed between the separators for supplying a coolant along the surfaces of the separators. Generally, each of the fuel gas flow field, the oxygen-containing gas flow field, and the coolant flow field includes a plurality of serpentine grooves or straight grooves connecting a fluid supply passage and a fluid discharge passage extending through the separators in the stacking direction.
When the size of the fluid supply passage or the fluid discharge passage connected to the grooves is small, it is required to provide a buffer section around the fluid supply passage or the fluid discharge passage for preventing closure of the grooves by flooding of liquid flowing through the grooves.
For example, a gas flow field plate of a fuel cell as disclosed in Japanese laid open patent publication No. 10-106594 is known. Referring to FIG. 10, a gas flow field plate 1 having an oxygen-containing gas passage is provided. An inlet manifold 2 for the oxygen-containing gas is provided at an upper part of the gas flow field plate 1, and an outlet manifold 3 for the oxygen-containing gas is provided at a lower part of the gas flow field plate 1.
The gas flow field plate 1 includes an inlet flow field 4a connected to the inlet manifold 2, an outlet flow field 4b connected to the outlet manifold 3, and an intermediate flow field 5 connecting the inlet flow field 4a and the outlet flow field 4b. Protrusions 6a are provided in the inlet flow field 4a and the outlet flow field 4b. Thus, the inlet flow field 4a and the outlet flow field 4b are formed in a matrix pattern. The intermediate flow field 5 has a curved pattern including a plurality of turn regions. A plurality of straight grooves 7 and matrix flow fields 8 are provided in the intermediate flow field. The matrix flow fields 8 are formed by a plurality of protrusions 6b at the turn regions.
In the gas flow field plate 1 of the fuel cell, the inlet flow field 4a and the outlet flow field 4b function as buffer sections. A large area of the electrode surface is exposed to the gas supplied to the electrode, and the gas freely moves along the electrode. In the intermediate gas flow field 5, the reactant gas flows through the plurality of straight grooves 7 uniformly at high speed.
In the gas flow field plate 1, practically, a plurality of serpentine flow passages 1a are formed between the inlet manifold 2 and the outlet manifold 3. At the turn regions of the flow passage 1a, the flow fields 8 in the matrix pattern are formed by the protrusions 6b. The inlet flow field 4a and the outlet flow field 4b are formed in the matrix pattern by the protrusions 6a. 
The inlet flow field 4a, the outlet flow field 4b, and the matrix flow field 8 form buffer sections. In the buffer sections, the contact resistance increases, and the power generation performance decreases undesirably. In particular, the buffer sections take up a large surface area of the gas passage plate 1 having the flow passage 1a. Therefore, power generation may not be performed efficiently.